Add a github action on pull request to check lua formatting (nvim-lua#526)

GitHub Action used:
https://github.com/marketplace/actions/stylua

This runs `stylua --check` on a PR and it will show
success or failure. The suggested stylua changes can
be inspected when clicking on the details.

The PR can still be merged even if the check fails.

Author: dam9000

.github/workflows/stylua.yml

@@ -0,0 +1,18 @@
+# Check Lua Formatting
+name: Check Lua Formatting
+on: pull_request
+
+jobs:
+  stylua-check:
+    name: Stylua Check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v2
+      - name: Stylua Check
+        uses: JohnnyMorganz/stylua-action@v3
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version: latest
+          args: --check .
+